In general, in a uniaxial eccentric screw pump, a stator expands and contracts in response to a change of a liquid temperature or an atmospheric temperature. Accordingly, there may be a case where it is difficult to convey a fluid material in an appropriate state which corresponds to such a change. For example, in CIP (Cleaning In Place) or SIP (Sterilizing In Place), vapor or hot water of a high temperature is made to flow in the pump and hence, the above-mentioned problem arises. That is, in CIP or SIP, after a fluid material (foods, chemicals or the like) of a room temperature is conveyed, vapor or hot water of a high temperature is made to flow in the pump for cleaning or sterilizing the inside of the pump. At this stage of operation, when an interference between a rotor and a stator is set to a value which conforms to a fluid material of a room temperature, the stator expands so that an interference becomes excessively large whereby a frictional force between the rotor and the stator becomes large. As a result, a torque required for rotating the rotor is increased or the stator wears or is damaged earlier than an expected life time. On the other hand, when an interference between the rotor and the stator is made small in advance by taking into account the expansion of the stator brought about by vapor or hot water of a high temperature which is made to flow in CIP or SIP, a fluid material of a room temperature cannot be properly conveyed.
Conventionally, as a uniaxial eccentric screw pump capable of solving such a problem, there has been known a uniaxial eccentric screw pump having the following configuration. In such a state where a stator made of an elastic material is housed in the inside of a casing and a rotor is inserted into the stator, an air pressure in a space formed between the casing and the stator is adjusted so as to elastically deform the stator toward the inside thus maintaining a contact pressure of the stator to the rotor at a fixed value (see JP 60-173381 A, for example).
However, it is difficult to properly control a pressure in the space for maintaining a contact pressure of the stator to the rotor at a fixed value. When the pressure is large, for example, a cavity which is a space for conveying a fluid material formed between the rotor and the stator becomes small so that the pump cannot acquire a desired discharge amount. Further, a frictional force between the rotor and the stator becomes large so that a torque required for rotating the rotor is increased or the stator wears earlier than an expected life time. On the other hand, when the pressure is small, even when the rotor is rotated, a sufficiently smooth flow of a fluid material cannot be ensured whereby it is impossible to discharge the fluid material at a desired discharge pressure.
Further, an air pressure directly acts on the stator and hence, when damage such as a crack occurs in the stator, there may be a case where air leaks through a damaged portion. In this case, it is difficult to press the stator to the rotor at a desired fastening pressure. There may be also a case where air is mixed into a fluid material through the damaged portion or a fluid material flows out through the damaged portion. Mixing of air into a fluid material (particularly food) gives rise to a problem in terms of quality. On the other hand, when a fluid material flows out to the surrounding, a flowout portion of the surrounding is contaminated.